1. Field of the Invention
The present invention relates to an image bearing member, image forming apparatus and process cartridge.
2. Discussion of the Background
Recently, image bearing members have been actively developed using organic photoconductive materials as photoconductive materials for use in image bearing members, which have advantages over inorganic materials such as Se, CdS and ZnO in terms of sensitivity, thermal stability and toxicity. Image bearing members formed of such organic photoconductive materials have been provided in a number of photocopiers and printers. When a photosensitive layer of an image bearing member formed of such organic photoconductive materials is formed, a function separation type in which a charge transport layer is accumulated on a charge generating layer is widely used because the function separation type has excellent sensitivity and durability. Generally, in an image forming apparatus such as printers, photocopiers and facsimile machines, images are formed through the series of repeated processes of charging, irradiating, developing, transferring and fixing. In recent years, with the advance of speed and durability of electrophotographic photocopiers, image bearing members have been demanded to have a high reliability such that quality images can be produced for repetitive use for an extended period of time. Especially, since the copy volume is large in the case of an ultra high speed photocopier, the image formation thereby is relatively frequently suspended by replacing the image bearing member, which causes significant decline of the productivity. In the case of color image formation, a tandem system in which 4 color developing systems are arranged is popularly diffused. To avoid the size increase of a photocopying machine, an image bearing member having a relatively small diameter is used. Therefore, such an image bearing member is demanded to have a higher durability corresponding to the speed-up of image formation.
With regard to the durability, one of abnormal images ascribable to an image bearing member is background fouling in the currently dominant image formation system, i.e., negative positive development. Specific causes of background fouling are, for example, the contamination and deficiency of an electroconductive substrate, the electric insulation breakdown of a photosensitive layer, the infusion of carriers (charge) from a substrate, the increase of dark decay of an image bearing member, and the thermal generation of carriers. Among these, it is possible to deal with the contamination and deficiency of an image bearing member by eliminating such a substrate before applying a photosensitive layer thereto. Since this is caused by an error in a sense, this does not make an essential cause. Therefore, it is thought that the background problem can be fundamentally solved by improving the property of anti-dielectric breakdown of an image bearing member and preventing the charge infusion from a substrate and electrostatic fatigue of an image bearing member.
In consideration of these, technologies such that an undercoating layer or an intermediate layer is provided between an electroconductive substrate and a photosensitive layer have been proposed in the past. For example, JOP S47-6341 describes an intermediate layer containing a cellulose nitrate based resin, JOP S60-66258 describes an intermediate layer containing a nylon based resin, JOP S52-10138 describes an intermediate layer containing a maleic acid based resin, and JOP S58-105155 describes an intermediate layer containing a polyvinyl alcohol resin. However, such a single intermediate layer formed of a simple resin has a high electric resistance, which causes the residual potential to rise. As a result, the image density deteriorates in a negative positive development.
In addition, such an intermediate layer shows ion conductivity caused by impurities. Therefore, the electric resistance of the intermediate layer is extremely high in a low temperature and low humid circumstance. This extremely raises the residual voltage. Therefore, it is necessary to make the thickness of an intermediate layer thinner, which causes a drawback that the charging property is not sufficient after repetitive use.
To deal with these problems, JOP 2002-131961 describes a technology to control the electric resistance of an intermediate layer in which an image bearing member having an intermediate layer containing a thermosetting resin and a specific contact angle is provided. Further, as a method of adding a conductive additive to an intermediate layer bulk, JOP S51-65942 describes an intermediate layer in which carbon or chalcogen based material is dispersed in a curing resin, JOP S52-82238 describes a thermopolymeric intermediate layer in which a quaternary ammonium salt is added and an isocyanate based curing agent is used, JOP S55-113045 describes a resin intermediate layer in which a resistance controlling agent is added, JOP S58-93062 describes an intermediate resin layer in which an organic metal compound is added, and JOP H04-269761 and H10-268543 describe an intermediate layer in which a cross linking agent is contained in a polyamide resin. However, there is a problem that, when these single intermediate resin layers are used in an image forming apparatus of late years using coherent light such as a laser beam, moiré is observed in images obtained.
Further, to prevent moiré and control the electric resistance of an intermediate layer at the same time, an image bearing member having a filler in its intermediate layer is proposed. For example, JOP S58-58556 describes an intermediate resin layer in which an oxide of aluminum or tin is dispersed. JOP S60-111255 describes an intermediate layer in which electroconductive particles are dispersed. JOP S59-17557 describes an intermediate layer in which a magnetite is dispersed. JOP S60-32054 describes an intermediate resin layer in which titanium oxide and tin oxide are dispersed. JOPs S64-68762, S64-68763, S64-73352, S64-73353, H01-118848 and H01-118849 describe an intermediate resin layer in which powder of borides, nitrides, fluorides and oxides of calcium, magnesium, aluminum, etc., are dispersed. JOPs 2001-209200 and 2003-98705 describe an intermediate layer in which two kinds of organic particles having a different average particle diameter are dispersed.
To have suitable electric characteristics by the filler dispersed, such an intermediate layer in which a filler is dispersed contains the filler in a large amount, that is, the amount of a resin contained therein decreases. Therefore, there is a problem that, as the content of a resin decreases, the adhesive property between the intermediate layer and an electroconductive substrate deteriorates, which easily causes detachment thereof. Especially, this has a significant adverse effect on an image bearing member formed of an electroconductive substrate having a flexible belt form.
To deal with these problems, a technology of a layered intermediate layer is proposed. Largely, there are two layered types. One is that a resin layer 102 in which a filler is dispersed and another resin layer 103 in which a filler is not dispersed are disposed on an electroconductive substrate 101 in this order (refer to FIG. 1). The other is that the resin layer 103 in which a filler is not dispersed and the resin layer 102 in which a filler is dispersed are accumulated on the electroconductive substrate 101 in this order (refer to FIG. 2).
The former structure is detailed as follows. To seal off the deficiency mentioned above involved in a substrate, an electroconductive layer in which a filler having a low electroconductivity is dispersed is provided on an electroconductive substrate. Further, the resin layer mentioned above is provided on the electroconductive filler dispersed layer. For example, JOPs S58-95351, S59-93453, H04-170552, H06-208238, H06-222600, H08-184979, H09-43886, H09-190005, and H09-288367 describe such a structure.
Since the electroconductive filler dispersed layer disposed as the bottom layer in this structure functions as an electrode of the electroconductive substrate in an essential sense, the electrostatic problem involved in the image bearing members having a single intermediate resin layer mentioned above still remains. But the electroconductive layer is formed of a filler dispersed film and can scatter writing beam so that moiré can be prevented. In this structure, since the bottom layer is an electroconductive layer, charges reversely charged to the polarity of the surface of an image bearing member during charging can reach the interface between the bottom layer (electroconductive layer) and the top layer (intermediate resin layer) Thereby, the image bearing member can function. However, when the resistance of the electroconductive layer is not sufficiently low, the charge infusion from the electrode is not sufficient, either. Therefore, such a bottom layer can be a resistance component during repetitive use, which causes significant increase of the residual voltage. This problem has a significant meaning because it is necessary to make the bottom layer sufficiently thick (not less than 10 μm) to seal off the deficiency of an electroconductive substrate, which is one of the objects of this structure.
To the contrary, the latter structure is a structure in which a single positive hole blocking resin layer is provided on an electroconductive substrate and a resin layer is provided thereon in which a filler having a low resistance or an electroconductive filler is dispersed as described in JOPS H05-80572 and H06-19174. This structure has the same positive hole blocking function as the former structure and therefore is effective against the background fouling. In addition, since the top layer is a filler dispersed layer, the accumulation property of the residual voltage is relatively low in comparison with that of the former structure. As mentioned above, this structure can prevent charge (positive hole) infusion from an electroconductive substrate to a photosensitive layer so that the background fouling phenomenon in the negative positive development can be significantly reduced. Further, by disposing a charge blocking layer as the bottom layer, the rise of the residual voltage is relatively small during repetitive use in comparison with when a charge blocking layer is disposed as the top layer.
An image bearing member having a layered intermediate layer formed of a charge blocking layer dispersed as the bottom layer and a filler dispersed layer as the top layer is effective to prevent the background fouling as mentioned above. However, such an image bearing member has a problem in that the charging responsibility thereof decreases after repetitive use over an extended period of time and the charging voltage is low only during a first rotation of the image bearing member. Thereby, the background fouling ascribable to the low charging voltage occurs at the top portion of a first output image. As a method adopted for avoiding this phenomenon, the first rotation of an image bearing member is not for use in image formation and images are formed when and after the image bearing member rotates twice. As another method, JOP 2002-268335 describes a method of providing a preliminary charging process. Naturally, these methods involve problems such as the speed-down of image formation and the size increase of a device and are therefore not suitable in terms of long durability required along with demands of late, i.e., the reduction of the diameter of an image bearing member and speed-up of an electrophotographic device.